1. Field of the Invention
This invention relates in general to time division multiplex digital switching systems, as for example, for PCM telephone exchanges utilizing two or more coupling field installations.
2. Description of the Prior Art
Recent switching technologies has led to the development of switching systems wherein the transmission of time continuous analog signals in transmission channels which are spacially separated from one another is not utilized as in conventional telephone communication systems particularly telephone exchange systems, but on the other hand, instead of space multiplex techniques, time division multiplex techniques are utilized whereby chronologically discontinuous analog or digital signals are transmitted.
In recent telecommunications systems, particularly telephone exchange systems, there has been a tendency towards centralization of the equipment and there has been a tendency towards concentration of all functions which can be together observed in a large switching center or at a few large central installations. A concentration of functions such as has occurred inevitably requires a great degree of dependability on the operability of the entire switching system, so as to assure the operability of such one or more less central installation systems, however, the possibility of failure can never be completely prevented. So as to provide for the contingency of a possible failure, installations can be provided in duplicate, whereby the respective reserve installation constantly operates in parallel with the actual operational installation and which upon failure of the operational installation can be put in operation to replace it in the train of a so-called standby or replacement apparatus. For time division multiplex switching network, it is known from the introduction of the specification of German Auslegeschrift No. 1,243,734, to provide in duplicate time-division multiplex lines connected to one another by way of a coupling field of the time division multiplex center and to provide in duplicate the coupling field of the time division multiplex switching network, so as to be able to eliminate failures on the time division multiplex line which are connected with a failure of an entire subscriber termination or connection group. It is also known from German Pat. No. 2,320,123 for bringing together the time division multiplex lines of doubly provided coupling fields of time division multiplex digital telecommunications switching networks such as PCM telephone exchange centers, to connect mutually corresponding output multiplex lines of duplicate time division multiplex networks to the two inputs of two change-over switching installations, which are respectively each connected to a switching network and the change-over switching networks respectively give priority to the time channel signals contained in their own coupling field and which are connected by way of a following common change-over switching network to the outgoing multiplex line. The common change-over switching network insures that only one of the two first change-over switching networks is operative or effective in each instance, whereby the respective coupling field operates in a master fashion, whereby the other coupling field operates in a slave fashion.